Generally, discharge bulbs, particularly high-intensity discharge bulbs (HID bulbs) such as metal halide bulbs, high-pressure sodium bulbs and mercury bulbs have advantages of large luminous flux, high lamp efficiency and long life. Accordingly, they have been used as illumination lamps of indoor and outdoor facilities, warehouses and factories, and as streetlights. In particular, they are now employed as headlights of vehicles such as automobiles.
An ordinary HID bulb utilizes gases such as xenon, metal halides and mercury in the bulb to stabilize its interelectrode voltage (bulb voltage) at a specified voltage during discharging and lighting, thereby stabilizing its luminous quantity. For example, a mercury-in-bulb, onboard 35 W HID bulb (the so-called D1 and D2 types are typical) can stabilize the bulb voltage at 85 V during stable lighting. Because of these advantages, most of the HID bulbs utilize mercury in general under the present circumstances.
However, since the mercury, which is sealed into the bulb to stabilize the bulb voltage, increases the environmental load, HID bulbs without using mercury have been studied, as disclosed in Japanese patent application laid-open No. 11-86795/1999, No. 2002-110099, No. 2002-93368 and so on. Likewise, as for the onboard HID bulbs, HID bulbs without using mercury (the so-called D3 and D4 types are typical) have been proposed.
The onboard HID bulbs without using mercury (D3 and D4 types) utilize zinc or indium instead of adding mercury. The zinc or indium, however, requires more amount of heat for evaporation than mercury.
In addition, as for 35 W HID bulbs with the same rating as the conventional HID bulbs (D1 and D2 types), although the D1 and D2 type HID bulbs have the bulb voltage of 85 V during the stable lighting, the D3 and D4 type HIDs have the bulb voltage of 42 V during the stable lighting.
Furthermore, the behavior immediately after switching on the light is different. For example, the voltage immediately after turning on the HID bulb is determined by the components and pressure of the gases in the HID bulbs. As for the conventional HID bulb using the mercury, the mercury, which quickly becomes gaseous, brings about a voltage drop. Thus, the HID bulb increases the voltage rapidly, and the gaseous mercury emits light by itself, thereby increasing the luminous quantity quickly. In addition, regulating the output power to be supplied to the HID bulb in accordance with the HID bulb voltage or lighting elapsed time makes is possible to produce a constant luminous quantity.
However, as for the HID bulb without using the mercury, since it does not include mercury for bringing about the voltage drop, only a xenon gas is present in the HID bulb until the metal halides vaporize immediately after switching on the light. Therefore the HID bulb without using the mercury has a nearly constant, low voltage immediately after switching on the light because of the voltage drop due to only the xenon gas. In addition, since the quantity of light is limited to that emitted by the xenon gas, the luminous quantity is comparatively low for the power at turn-on.
Consequently, when the HID bulb such as the HID bulb without using the mercury (D3 and D4), which requires a large amount of heat until the metals in the HID bulb vaporize, is turned on by a discharge bulb ballast for the conventional HID bulb containing the mercury, the output power can attenuate before the metal halides evaporate, that is, before the metal halides start to emit light. As a result, the HID bulb cannot be supplied with appropriate power, and takes a considerable time until it produces a sufficient luminous quantity. In particular, since the onboard headlights require sharp start-up of the luminous quantity, it is difficult for this purpose to use the conventional discharge bulb ballast. In addition, when fillers vaporize, the luminous quantity increases sharply, which presents a problem in that the luminous quantity at the lighting up of the discharge bulb is unstable.